Canine transmissible venereal tumor: The 11,000-year old dog cancer

The canine transmissible venereal tumor is a rare type of cancer that affects dogs. Also referred to as canine transmissible venereal sarcoma, sticker tumors, and infectious sarcoma, this disease affects mainly the external genitalia of dogs and in some rare cases, their mouth or nose. The cancer results in the appearance of a grotesque-looking, cauliflower-like tumor.

When affecting the penis or prepuce of male dogs, or the vagina or labia of female dogs, the review study of Utpal Das and Arpu Kumar Das mentioned that the disease can cause discharge and urinary retention. When affecting the facial area, the study of L. G. Papazoglou et al revealed that the disease causes nosebleeds and other nasal discharge, fistulae, facial swelling, and/or enlargement of the submandibular lymph nodes.

Although canine transmissible venereal tumor affects a considerable number of dog populations across the world, it remains rare simply because it is a transmissible disease. What is more interesting to note is that all dog suffering from this disease have acquired cancer cells that descended from a single or several cancerous cells that emerged in a single ancestral dog about 11,000 years ago.

Understanding transmissible cancer in dogs

Cancers typically emerge when a single cell in the body acquires mutations that result in uncontrolled cellular division that further leads into emergence of tumors. These cells spread to different parts of the body through a process called metastasis. As a disease, cancer is not normally contagious. It is very rare for cancer cells to leave the bodies of their original hosts and to spread to other individuals. However, an extremely rare type of cancer can actually be contagious.

There are currently four known cases of parasitic or transmissible. These are the devil facial tumor disease affecting Tasmanian devils, contagious reticulum cell sarcoma affecting Syrian hamsters, leukaemia affecting soft-shell clams, and the canine transmissible venereal tumor affecting dogs.

In transmissible cancers, the cancer cells are themselves the infectious agents and the tumors that form are not genetically related to the affected organism. Thus, in the case of a dog suffering from the canine transmissible venereal tumor, the disease is a result of external cancer cells entering its body and affecting its organs and tissues.

Canine transmissible venereal tumor may be transmitted through coitus or sex, or by licking, biting, and sniffing areas affected by the tumor. It is important to note that the cancer cells can also be transmitted and thereby, infect other canid species to include foxes and coyotes. The review study of S. Mukaratiwa and E. Gruys further added that the disease is the only known naturally occurring type of cancer that can be transplanted as an allograft across major histocompatibility barriers within the same species and even in other members of the canid family.

Mukaratiwa and Gruys added that the progression of the cancer follows a predictable growth pattern observed both in natural and experimental cases. The cancer would enter an initial growth phase lasing for four to six months, followed by a stable phase, and a regression phase. Metastasis is a rare occurrence except for puppies and immunocompromised dogs.

The oldest continuously surviving cancer

Researchers Clare A. Rebecca et al mentioned that the cancer cells are now collectively and essentially living as a unicellular, asexually reproducing pathogen. As mentioned, the particular cancer cell is an infectious agent and the tumor it creates is genetically unrelated to the affected host organism.

Through a genomic sequence analysis, Rebecca et al concluded that the original cancer cell emerged from a dog or wolf more than 6000 years ago. However, the study of Andrea Strakova and Elizabeth P. Murchison further revealed that the cancer cell first emerged about 11,000 years ago from the somatic cells of an individual dog. The cell acquired several adaptations or mutations that made it transmittable between hosts while also acquiring a genome configuration that makes it compatible for long-term survival.

Another study conducted by Elizabeth P. Murchison et al provided several hypothetical insights. Probably, the first cancer cell survived the death of the individual ancestral dog particularly through sexual transmission. In addition, through a genomic sequence analysis of cancer cell samples obtained from different continents, patterns of genetic variants suggested that the cancer cells existed in one isolated population of dogs for most of its history. The global transmission was likely to have occurred within the last 500 years, possible during the age of exploration when seafarers and explorers had brought in dogs to their global expeditions.

The genomic sequence analysis performed by Murchison et al further suggested that the first ancestral dog that hosted the cancer cell might have resembled an Alaskan malamute or a Husky. It probably had a short, straight coat that was coloured either grey and brown or black. Although the researchers were unable to determine the biological or even environmental circumstances that gave rise to a transmissible cancer, it is interesting to take note of the fact that the genome of the ancient and ancestral dog is still alive today in the cells of the cancer it spawned.

It is also worth mentioning that the genomic sequence analysis performed by Murchison et al revealed that the present cancer cells carry about two million mutations. Note that human cancer cells carry between 1000 to 5000 mutations.

Nonetheless, apart from the fact that it is one of the rare types of transmissible cancer, the canine transmissible venereal tumor remains intriguing and significant because it is the oldest continuously surviving cancer that has extensively spread and thereby, affected dogs across the world.